Contact lenses were described by Leonardo da Vinci as early as the 1500s. Early contact lenses were made from optical glass between 1920 and 1940. These lenses were prescribed by the Royal Air Force in Britain during World War II to try to increase the number of pilots by correcting the vision of otherwise acceptable candidates. Unfortunately, these contact lenses were uncomfortable to wear and caused significant eye irritation. Such factors resulted in short wearing times.
Early contact lens designs fused two materials—glass over the cornea and plastic over the scleral portion of the eye. Various configurations, including channels for tears and air availability to the cornea, were tried in order to improve comfort and wearing time. Transparent plastic was introduced in the mid-thirties and became the primary material in contact lenses. As development progressed the lenses became smaller as it was discovered that a corneal contact lens was all that was required to properly correct a user's vision.
Comfort increased dramatically with the introduction of hydrogel contact lenses. These lenses have been available for three decades and have undergone considerable improvement. Initially, the lenses were suitable only for patients with a minimal amount of astigmatism, since the first soft contact lenses did not correct for astigmatism. Later developments provided lenses with weighting systems or thicker portions to force the lens to rest on the eye in the orientation required to correct the patient's astigmatism. Further developments have led to the development of bifocal and multifocal contact lenses for patients with presbyopia.
All of the soft contact lenses described above have spherical and/or aspherical posterior surfaces with one or more zones. In each case these lenses have utilized a central radius of curvature that is flatter than the central radius of curvature of the eye to which the lens is fitted. In addition, the total sagittal depth of the lens and the volume of the lens exceed the sagittal depth and volume of the eye to which the lens was fitted. As a result, the lens deforms on the eye.
Lens deformation plays a significant role in the comfort and visual acuity of the wearer. Hydrogel materials change dimensionally as the water content varies, particularly as the lens becomes drier. These environmentally induced dimensional changes result in a decrease in the radial and linear dimensions as the water content decreases. Lenses with a sagittal height and volume greater than that of the underlying eye decrease in radius and thereby increase in sagittal height as the water content decreases during wear. This change induces an increased deformation as the dimensions of the lens move further away from the dimensions of the eye fitted for the lens.
Currently, work is underway to produce lenses providing higher order aberration correction for the purpose of allowing the human retina and brain to achieve maximum visual acuity. Higher order aberration corrections often deviate only fractions of a micron from a sphere over the diameter of the eye pupil. The magnitude of change in contact lens optics due to lens deformation easily exceeds the magnitude of the correction for higher order aberrations.
A non-deforming contact lens must accommodate the full corneal diameter as well as the region of the sclera over which the lens is fitted. This requires that the corneal surface be measured precisely. Recent efforts have been made to use corneal topography to design contact lenses. These efforts have been limited by the size of the cornea that can be measured with computer assisted video keratography. The average diameter that can be measured by placido systems is approximately 9 mm.